The invention relates to a force sensing means comprising a capacitor arrangement of at least two capacitor plates and a compressible dielectric.
Capacitive force sensing means with compressible dielectric have been developed because they can be made from homogeneous layers of cheap material. Thus, the labor and material costs are low and no mechanical precautions are necessary for taking into account the forces applied or for storing the material.
When such force sensing means are employed, very considerable errors in measurement occur, due to such factors as non-linearity, hysteresis, symptoms of relaxation, material fatigue, low flexibility. The force sensing means are usable only on hard flat surfaces and have short mechanical durability as well as a mechanical coupling to adjacent capacitors, such as in the force sensing means disclosed in the German published application DE-OS No. 25 29 475.
For reducing the non-linearity, the force sensing means disclosed in German Allowed application DE-AS No. 2 448 398 employs, for example, a nap-shaped dielectric and thus makes transverse extension under load possible.
Many arrangements of the capacitor plates have been known for a long period of time. For example, the plates have been formed from strong stiff sheets to achieve stiff arrangements. To make the arrangements flexible, thin sheets or metallized films have been employed. Moreover, lattice electrodes have been used to make the plates especially flexible.
To achieve force sensing means which will match curved and time-variable surfaces, it has been proposed in German published application DE OS No. 25 29 475 to coat both surfaces of the dielectric with systems of intercrossing strips of conductive material. When the force sensing means bends, the material on one surface is upset and the material on the opposite surface is extended. This occurs because the distance between the strip systems adapts itself to the conditions. The extensibility in the direction of the strips is however not increased over the normal extensibility.
For a large number of applications, particularly for measurements with respect to biological objects, the following considerations are of special importance:
1. the hysteresis, the magnitude of which is responsive to the deflection and which is therefore difficult to take into consideration.
2. the requirement, when biological objects are measured, that an automatic mechanical adaptation to the object is necessary which adaptation must also take place when the object varies its form during measurement. This adaptation requires that parts of the measurement receiver be extensible with respect to length.
The problem underlying the invention is to develop a capacitive force sensing means having a very small hysteresis and being adapted for use without special mechanical adaptation under most different circumstances including curved and time-variable measuring bodies.
To solve these problems, the force sensing means of the present invention employs capacitor plates comprising a metallic cloth, the warp threads and the weft threads of which define an angle which easily accommodates external forces and the dielectric is a macroscopically homogeneous foam elastomeric material, such as foamed elastomeric polyurethane or a foamed mixture of natural rubber and polystyrene butadiene rubber.
Macroscopically homogeneous, foamed elastomeric polyurethane and a foamed mixture of natural rubber and polystyrene-butadiene rubber are subject to a small amount of hysteresis.
The use of metallic cloth for capacitor plates has the advantage that the adhesive used for fixing the layers is pressed into the interstices in the cloth so that the cloth lies directly upon the dielectric and inhomogeneous interfaces producing hysteresis are not developed.
In order to adapt receivers automatically to curved and time-variable bodies, it is advantageous to 1. form the capacitor plates as in DE-OS No. 25 29 475 into strips and 2. to make said (electrically conductive) strips extensible. This is achieved by arranging the strips to form a metallic cloth, the angle between the warp threads and the weft threads being easy to influence by external forces. When the warp threads or the weft threads extend parallel to the direction of the strips, parallel threads can be shifted with respect to each other in longitudinal direction and the length of the strip is changed thereby. When the direction of the threads is not parallel to that of the strips, the length may be varied by changing the shape of the rectangles formed by the threads to parallelograms.
Extremely small hysteresis may be achieved particularly by polyurethane having a low degree of foaming. It is true that the low compressibility connected with low foaming degree of the capacitor formula EQU C=.epsilon..epsilon..sub.o F/d
(wherein C=capacitance, .epsilon..sub.o =absolute dielectric constant, .epsilon.=relative dielectric constant, F=capacitor surface, d=distance between the plates) results in only a small signal portion depending on the change of the distance between the plates. This drawback is, however, partly compensated by the high relative dielectric constant of about 7. Moreover, mechanical irregularies in the compression (such as irregular deformation, bursting or collapsing of the cell walls) occur only when the pressure increases, which is advantageous particularly with respect to the problem of hysteresis.
In order to avoid electric noise pick-up, it is advisable to provide additional insulating layers and additional capacitor plates which may be used as shields. In a device like that disclosed in DE-OS No. 25 29 475, this configuration is advantageous for another reason. The dielectric in this arrangement is coated on both surfaces with intercrossing strip systems of conductive material and at each cross point of two strips a capacitive force sensing means is defined. Thus, distribution of force can be measured with low expenditure. In this arrangement, the precision capacitor can be connected in series with a fixed capacitor and both can be connected to a generator so that between the two capacitors a power-responsive voltage can be measured. In this case, the fixed capacitor is formed economically by the additional insulating layer and the additional capacitor plate. The dependent claims 2 to 20 relate to advantageous structures of the insulating layers as well as of the capacitor plates which are conductive layers.
These embodiments are directed particularly at achieving greater extensibility of the layers. Therefore, even if only force, and not the distribution of force, is to be measured, it may be appropriate to cut large-area layers into strips which are connected electrically. It is likewise advantageous to make the capacitor plates of metallic fabric wherein the angle between the warp threads and the weft threads can be influenced easily by exterior forces, and not to cut the strips parallel to one direction of the thread of the metallic fabric since the extensibility of the fabric can thus be increased considerably. In the case of a cut to provide a 45.degree. angle between the threads and the longitudinal dimension of the strip, the rectangles formed by the threads of the fabric are deformed in diagonal directions to parallelograms due to tensile stress and allow extensions of a multiple of ten percent. This is advantageous to facilitate the employment of the force sensing on uneven bodies or on bodies which are deformed during the measurement. According to DE-OS No. 25 29 475 the mechanic coupling with adjacent devices in arrangements on a hard base is reduced by extensible strips. For example, in the case of a lumped load an extensible strip adapts itself to the enlarged surface of the cone of deformation.
In the case of narrow strips and a cutting angle of 45 degrees the strip resistance increases considerably. The resistance can be reduced -
by a cutting angle between 45 and 0 degrees depending on the width of the strip wherein a compromise is made between the requirements of extensibility and conductivity;
by strips of threads extending zigzag through the strips; or
by coating the metallic fabric with flexible conductive lacquer or conductive glue. The only advantage of this method over the capacitor plates or conductive layers provided with conductive lacquer or conductive adhesive is the higher mechanical stability.
When the extensibility or flexibility of the force sensing means is not required, it is advantageous to make the non-conductive incompressible layer at least partly of plexiglass and to glue the force sensing means to an iron sheet to improve the mechanical stability.
For insulation on the outside of the measuring means, and for mechanical protection an additional insulating layer is applied on every outermost capacitor plate. This layer should be designed so as not to have a substantial effect on the extensibility or flexibility of extensible or flexible force sensing means. This problem is solved by employing small pieces of flexible material which overlap one another like tiles.
Advantageous methods of building up the capacitive force sensing means are described in claims 24 to 27. The adhesive must also be flexible and extensible, which properties are fulfilled to a high degree by the contact adhesive. It is applied normally on both surfaces to be glued. The surfaces--after being aired--are glued together. This technique requires homogeneity of the adhesive layer since inhomogeneities of the layer entail inhomogeneities in the basic capacity and in the force sensitivity. However, if one of the layers to be glued is a metallic fabric, the fabric can be applied on the wet adhesive layer. The adhesive which may be applied inhomogeneously is pressed through the mesh of the fabric and becomes electrically ineffective; the adhesive can be aired through the mesh of the fabric. The metallic fabric may be bonded to insulating layers by applying cyanoacrylate adhesive on the surface of the fabric which is averted from the insulating layer.
When the force sensing means is to be placed on a severely bent hard body, the force sensing means may be overstrained with respect to flexibility and extensibility. In such a case, the force sensing can be glued to the body in layers and adapted to the shape of the body. The use of contact adhesives is recommended as in the manner used to coat self-adherent films since then the sensing means can be removed in layers after use and can be used repeatedly.
When the strips have a width of a few millimeters, problems may arise with respect to production accuracy. It is advantageous to glue, for instance, the three materials used for the strips, then cut them to strips and then glue the finished strips on the dielectric. An alternative method would be to glue the dielectric prior to cutting the strips and to glue the 4-layer strips thus prepared on the second strip system.